This invention relates to fluid control valves, and in particular to a sealing structure which provides for sealing of such valves in the event of a fire or like occurrence.
Ball valves and butterfly valves used in regulating the flow of fluids commonly incorporate elastomeric sealing elements in their structure. For example, butterfly valves have a circular fluid flow channel therethrough and a circular disc mounted in the flow channel for rotation between an open position in which is substantially parallel to the flow of fluid through the valve and a closed position in which the disc is in a position substantially perpendicular to fluid flow. Typically, the disc edge contacts a relatively soft elastomeric annular seat positioned about the circumference of the fluid flow channel when the valve is in a closed position, and the sealing contact between the disc and annular seat will shut off the flow of fluid through the channel.
Such valve seats are commonly fabricated from such materials as fluorocarbon polymers, polyurethanes, and various olefinic polymers and copolymers. However, even the more thermally stable of these materials will fail if the valve is exposed to temperatures in excess of about 650.degree. F. If, for example, an accident occurred which led to the outbreak of a fire in the vicinity of such a valve, because of the low heat resistance of the valve seat materials, they could be damaged to a point where their condition would be tantamount to the absence of a sealing element in the valve. If the material transported through the valve were flammable, such as petroleum hydrocarbons, leakage from the valve could contribute to the seriousness of the accident.
It is because of this that valves having elastomeric seats have not been used to transport certain materials. Recently, however, fire-tested valves have been developed which function as conventional valves having elastomeric seats to seal the valve in general service but which have features which ensure that the valve will still seal properly if exposed to a fire or extreme heat. For example, Japanese Patent Publication No. 45-32861, published Oct. 22, 1970, discloses a butterfly valve structure having an annular metal ring which is held in a retracted position by a low-melting temperature alloy. When exposed to temperatures high enough to cause damage to the elastomeric seal, the alloy melts and the annular metal ring is released to form a metal-to-metal seal with the valve disc.
Likewise, U.S. Pat. No. 4,175,578 teaches a fire resistant seat for butterfly and ball valves having a prestressed annular metal collar adjacent a valve seat. When exposed to heat, the stresses stored in the collar are released, and the collar moves into sealing relationship with the valve element to prevent leakage. Finally, U.S. Pat. No. 4,202,365 discloses a fire-tested butterfly valve structure which has both an annular flexible metal seat and an annular resilient seat. The flexible metal seat is held out of sealing contact by either a fusible washer or an integral protrusion on the resilient seat. When the resilient seat is damaged by heat, the metal seat moves into sealing relationship with the butterfly disc to prevent leakage.
However, each of the above valve structure must rely on the performance of elements separate from the privacy sealing element at elevated temperatures to bring about the operation of the secondary metal sealing element. The alloy of the Japanese publication must melt at just the right temperature as must the fusible washer of the U.S. Pat. No. 4,202,365. The stored stresses in the metal collar of the U.S. Pat. No. 4,175,578 must be released and the taper of the collar designed so that it will function at the proper moment. Accordingly, the need still exists in the art for a valve structure which will be both simple and reliable in forming a secondary metal-to-metal seal upon high temperature deterioration of a primary sealing element.